Occasionally an author will see a listing for her book on a website she's had no previous contact with. Of course, the immediate question is, "Why are they selling my book? I never engaged with them to do so."

Chances are, they're not really selling it. When an author registers an ISBN with Bowker, that information goes into Bowker's Books in Print database. Many booksellers license BIP, and list the entire contents of it on their websites. They probably haven't made vendor agreements with many of the publishers in Books in Print. They're not stocking the books in their warehouses. If they receive enough orders for the book, they'll track down the publisher and arrange a vendor agreement.

So it's not generally a case of a website listing stolen goods for sale. It's because the website has received the information about the book (along with 28 million other books). If you see your book on a bookselling website, and you haven't made any sort of agreement with that company, you can do one of the following:

Contact the legal department of that website (preferably by email, so you have a written record of the request) and ask them to take the listing down.